Didn't I said Ash doesn't use his older pokemon before the League ! It doesn't maqtter how many pokemon he has in Professor Oak lad with the ability to megevolve if Ash doesn't call upon them before the league.
Also , Charizard is not Ash's signature Pokemon , he is his Secret Weapon !
If Ash going to get megevolution then he should get a signature pokemon as his Regular Megevolution partner.
There is rumor about a Big announcement in May 21st !
I predict it might be about the 3rd installment of Gen VII game.
So , Chances are that Kalo starter might get their Megevolution there ! In that case , Greninja will work better as Ash's regular Megevolution partner until Charizard appear.
After all, Greninja is popular ! I think I like it because his design remind me of ''Kamen Rider''
I hope Greninja's Megevolution will be like this--
An armored Ninja !